After bad tornado season, time to worry about hurricanes

It’s not because Mother Nature has gone wild on global warming juice. It’s a matter of simple odds.

The last hurricane to hit the state was Category 3 Wilma, which roared ashore near Naples and buzz-sawed across the peninsula, leaving a $9 billion trail of ripped roofs and shattered high-rise windows from Miami to Palm Beach. That was five years ago, come October.

History, the only reliable indicator of where hurricanes wind up, suggests South Florida is due.

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The statisticians at the National Hurricane Center calculate that the coastline from Palm Beach County to Key West has averaged a hit from a Category 1 hurricane every four to five years. It doesn’t take the sharpest knife in the drawer to figure South Florida’s hurricane-free run, at five years and counting, might just be at risk.

“Obviously, when you look at the return frequency, the greatest risk in Florida is South Florida,’’ said Dennis Feltgen, spokesman for the National Hurricane Center. “We’re sticking pretty far down into the tropics.’’

Most preseason forecasts predict a slightly calmer season than 2010, but that’s small comfort. Last year churned out 19 named storms — tied for third-highest number on record.

Defying the odds, none of the 12 storms that grew to hurricane strength made landfall on the mainland U.S. Last year also marked a record-tying fifth straight year in which the mainland has escaped a strike from major hurricane of Category 3 or above. But three storms did cause heavy damage and kill 250 people in the Caribbean and Central America.

Jerry Bell, lead seasonal hurricane forecaster for the National Oceanic and Atmospheric Administration’s Climate Prediction Center, said conditions that have seemed to super-fuel the tropics over the last decade remain largely in place. The brew includes warm Atlantic Ocean surface temperatures, running two degrees higher than normal, along with assorted favorable ocean and atmospheric conditions that have locked the tropics in a 17-year cycle of high activity. Eight of the 13 busiest hurricane years have been recorded since 2000, including 2005, with the all-time high for storms: 28.

The one change in global weather conditions, and a bit of a wild card, is the apparent waning of La Niña, a weather pattern marked by cooling temperatures in the eastern Pacific Ocean that typically tends to reduce wind shear, making it easier for storms in the Atlantic to form and strengthen. The expectation, said Bell, isn’t for a shift to an El Niño phase, which tends to knock back hurricane formation, but instead to something in between.

NOAA’s forecast calls for 12 to 18 named storms, including six to 10 hurricanes, with three to six developing into major storms — still “above-normal.’’

Where they will wind up is anybody’s guess. The steering currents that curved all 12 hurricanes away from the U.S. last year — the first time that has ever happened – are unpredictable, Bell said, and typically more variable than they were in 2010 .

There is at least one bit of upbeat news. Scientists say the record tornadoes and flooding devastating the South and Midwest aren’t harbingers of a cataclysmic hurricane season to come.

The powerful atmospheric forces generating those events aren’t big players in tropical storm formation, said Brian Soden, a climate researcher at the University of Miami.

“There is no real overlap,’’ said Soden, a professor of meteorology at UM’s Rosenstiel School for Marine and Atmospheric Science.

Like many scientists, he also cautions against viewing the record flooding and tornadoes as evidence that a warming world has suddenly flicked the “extreme’’ switch on the complex global weather machine.

The deadly weather has sparked debates in newspapers and blogs about what role climate change has played in the extreme weather events. Some environmentalists and scientists argue a hotter, moister atmosphere in the Gulf of Mexico has added fuel to the already volatile spring weather that typically produces the most intense twisters.

But Soden said it’s too soon to call the twisters as “a climate change signal.’’ That would be like projecting a baseball player’s production from the first few at-bats of a season, he said. It will take decades, he said, to measure how — and how much — influence climate change will have.

For instance, some initial research suggests hotter seas will produce more and stronger storms but subsequent studies suggest it could also create stronger wind shear that could shred hurricanes.

“The knee-jerk reaction is that it is going to lead to more hurricanes, stronger hurricanes, etc.,’’ Soden said. “The changes may be a mixed bag.’’

A preliminary assessment produced last month by what NOAA has informally dubbed its CSI team — short for Climate Scene Investigations — found nothing to indicate climate change played a role in the outbreak. Water vapor and wind shear, key ingredients in tornado formation, fell within ranges recorded over the past 30 years.

Martin Hoerling, a NOAA scientist who leads what is technically know as the Climate Attribution Rapid Response Team, said there was no disputing that temperatures have risen globally but measuring its impact at the local level is far more difficult and will require a concerted research effort.

“As we go to the local, we discover that the natural variability is much, much greater,’’ said Hoerling, who is based at NOAA’s Earth System Research Laboratory in Boulder, Colo. “We struggle to define what is natural variability and what is change.’’

It’s also unclear how still relatively minor climate changes could produce what he called “rogue’’ or “black swan’’ weather events — such as the 15 to 25 inches of rain recorded in the Mississippi Valley on April 14.

“It doesn’t mean climate change wasn’t a contributor,’’ Hoerling said. “We had twice as much rain as ever happened in this area and you can’t explain that from a 2 or 3 percent increase in moisture levels.’’

Jeff Weber, a scientist at the University Corporation for Atmospheric Research, which is affiliated with the National Center for Atmospheric Research in Boulder, said the explosion of twisters had been spawned by what he called a “classic set up’’ of the forces that fuel tornadoes.

One key was an atmospheric pattern called the North Atlantic Oscillation, which is a measure of the fluctuations between a low pressure system over Greenland and a high pressure one over the Azores. It can alter the alignment of the jet stream, which helps steer storms as they move across the country and influences weather in Europe and North America. Weber said they’ve been unusually persistent for the last 23 months, causing the jet stream to “buckle’’ and slowing storms. That allowed thunderstorms to slurp more moist warm air from the Gulf of Mexico and more cold air from the north — the perfect twister cocktail.

“I am a full believer in climate change and global warming. I can’t find any empirical evidence for it here,’’ he said. “’It’s not unprecedented. Sometimes, weather just happens.’’

The six-month hurricane season ends Nov. 30. The first named storm of 2011 will be Arlene.

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